It should probably be noted that you should NEVER use this tip for system updates unless you want to render your OS useless.
Of course, in general, you should always be careful deleting files you don't know much about. They may be there for other purposes for the files. Sometimes installers update files, so removing them will cause unintended consequences.

/Library/Receipts shows what's been installed; if you install Foo.pkg, you'll find something like /Library/Receipts/Foo.pkg/Contents/Archive.bom; to find out what it contains, do lsbom /Library/Receipts/Foo.pkg/Contents/Archive.bom.

The BOM won't indicate what files or folders were added, created, moved, or deleted by scripts. It also won't show what else has been done through scripts in the package — for example, I've seen installers which create users and groups.

The BOM will only list what files and folders were directly bundled together into the package. Therefore, using this tip, you can only see what was intended to be installed as part of the package, minus whatever actions were taken by any of the scripts in that package.

The Installer technology in Mac OS X is outdated and unacceptable in 2007. Not having an Uninstall feature is but one of its faults.

How about a Pre/Post script sandboxing API? There is simply NO reason that an installer script should have the full control over a system that a user holds, not by default.

How about a System-maintained Install history? TimeMachine is great, but a rather "dumb" technology. Context is key!

Unfortunately, Apple doesn't care about fixing this. Complaints about the Installer.app and its lack of feature-set have existed since 10.0, with no substantial changes. The fact that Windows provides, at least rudimentarly, a more robust solution is sad. Even Apple's pre-OS X installer tech was more full-featured.

IMHO the whole idea of having to "install" a program before I can use it feels impossibly quaint, after so many well-behaved apps that just let me drag a single app icon, or maybe a directory, out of the .dmg!

I groan every time a program requires me to run an installer, and if I don't feel it has good reason to need this, I'm as likely as not to just toss the installer unused. If you're not doing something really low-level, then you shouldn't be giving me an installer in the first place.

This shows you what file are "installed". It doesn't show you what files are created. By the application that is. And bets are even further off if there's any malfeasance involved. And don't forget it doesn't have to be malice - it can just be stupidity. This is an incomplete band-aid remedy for a complex issue.